1. Field of the Invention
This invention relates to the zincating of aluminum and metal plating of the zincated aluminum and, more particularly, to providing a metal plating pretreatment procedure for zincating aluminum to provide a plated aluminum product having smoothness and dimensional integrity of the aluminum substrate after plating with increased production yield of the plated products.
2. Description of Related Art
Metal plating of metals such as aluminum is of considerable commercial interest. One application, for example, is the preparation of aluminum substrate memory disks which are used in a variety of electronic applications such as computer and data processing systems. Aluminum is the preferred substrate for the disk although other suitable metals may be employed. The metal plating process for metals such as aluminum requires a lengthy and costly pretreatment process to prepare the aluminum surface for plating. The following will be directed to aluminum although it will be appreciated that other metals such as aluminum alloys, aluminum composites (e.g., containing boron carbide particles) may also be used.
In general in a typical metal plating on aluminum process, the ground aluminum substrate is first cleaned to remove dirt, grease and oils and then etched to provide a substrate surface suitable for adhesion of the zincate coating. The etched substrate is then desmutted with nitric acid to remove surface aluminum oxide and the aluminum substrate is then zincated followed by metal plating. For memory disks, an electroless paramagnetic nickel plating layer is plated and then finished with a sputtered cobalt or other magnetic layer. A double zincate procedure is typically used wherein a first zincate layer is stripped using nitric acid and then a second zincate layer applied to the aluminum substrate. The aggressiveness of the solutions used in the conventional process attacks the aluminum substrate and typically adversely affects the dimensional integrity and increases the surface roughness of the substrate and formed plated product.
Another problem associated with current metal plating on aluminum manufacturing processes is caused by the grinding process which is used to smoothen the aluminum substrate. During the grinding process cleaning agents are usually left on the substrate surface. The ground substrate is then typically annealed and the cleaning agents left on the surface tend to react with intermetallics within the substrate along with air, atmosphere and moisture to form metal oxides. Some of the oxides are not effectively removed by current chemistries and contribute to surface roughness.
As with all industrial processes, it is desired to improve the various steps of the process to enhance the overall efficiency of the metal plating on aluminum process. It is also highly desirable if any of the process steps can be deleted since this directly affects the cost of the process and the time required to complete the metal plating process. The smoothness of the final product may also be improved due to the fewer chemical solutions contacting the aluminum substrate.
For a memory disk application, a paramagnetic sublayer of electroless nickel phosphorus (ENP) is plated on the zincated aluminum and is used as the base for a thin layer of ferromagnetic material, i.e., Co, CoNiCr, etc. which is usually applied by sputtering. ENP deposits in excess of about 9% by weight phosphorus are paramagnetic as plated but these deposits lose their amorphous structure and become ferromagnetic above about 290.degree. C. Elevated temperatures on the order of 310.degree. C. can be reached during the sputtering process and at increasing temperatures even more thermally stable ENP deposits are required. By "ENP" is meant herein to be a electroless nickel deposit containing greater than about 9% by weight phosphorus but the invention is applicable to the metal plating of zincated aluminum substrates using other metals such as copper and the like.
The memory disk industry requires that the ENP deposit remain substantially nonmagnetic, e.g., less than 5 gauss (0.4 emu/cc) and preferably at its original level of less than 2 gauss (0.2 emu/cc) because if the deposit is ferromagnetic it would interfere with the read/write modes by diluting the signal and increasing noise levels.
This requirement has received attention in the industry and a number of articles have been written addressing enhancing the paramagnetic properties of the plated ENP by modifying the ENP bath or alloy composition. An improved method for depositing thermally stable ENP paramagnetic coatings is disclosed in U.S. Pat. No. 5,437,887, assigned to the assignee of the present application. Effective amounts of antimony and/or cadmium are used in the electroless nickel bath to provide the enhanced thermal properties.
While paramagnetic thermal stability of an ENP film is needed in the fabrication of memory disks, the demands of industry for memory disks and other metal plated zincated aluminum substrates have been changing resulting in even more stringent requirements for aluminum metal platers. The surface roughness of the metal plating is always important for a plater and is an especially important consideration in memory disks to achieve high magnetic density wherein more memory can be obtained for the same surface area for a smoother surface memory disk than for a rougher surface. Similarly, metal plated smoothness is likewise important for many products such as compressor vanes and electrical connectors.
For example, the aluminum substrate used to make memory disks previously had a roughness of about 1500 .ANG.. Aluminum substrates are now ground to a surface roughness of about 60 .ANG.or lower before fabrication into a memory disk. It is desired to maintain this low surface roughness in the formed ENP plated memory disk product, but as noted above, the disk manufacturing process involves an extensive pretreatment process to prepare the aluminum surface for plating. The pretreatment process typically roughens the surface due to aggressive etchants and/or zincating solutions which deposit thick, uneven zincate deposits.
Bearing in mind the problems and deficiencies of the prior art, it is an object of the present invention to provide a method for metal plating of zincated aluminum substrates.
Another object is to provide a method for fabricating aluminum substrate memory disks in which an electroless nickel-phosphorous (ENP) paramagnetic layer plated on the zincated aluminum has enhanced paramagnetic thermal stability due to the pretreatment of the disk.
Another object of the present invention is to provide metal plated aluminum substrates including memory disks fabricated using the method of the invention.
In a further object of the present invention a non-aggressive low aluminum etch method is provided for etching aluminum substrates, including aluminum substrates used for memory disks, to prepare the surface for zincating.
Another object of the present invention is to provide a non-aggressive low aluminum etch composition for etching an aluminum substrate, including, an aluminum substrate used for memory disks, to prepare the surface for zincating.
Another object of the present invention is to provide etched aluminum substrates made using the etching method of the invention which substrates are ready for zincating.
It is an additional object of the present invention to provide a method for zincating an aluminum substrate, including an aluminum substrate used for memory disk fabrication.
Another object of the present invention is to provide a composition for zincating an aluminum substrate including aluminum substrates used for memory disk fabrication to prepare the aluminum substrate for metal plating.
It is a further object of the invention to provide a zincating composition and method which provides enhanced smoothness and dimensional integrity of the aluminum substrate after plating with increased production yield of the plated product.
Another object of the present invention is to provide aluminum substrates, including aluminum substrates used for memory disk fabrication, made using the method and zincating composition of the invention.
Other objects and advantages will become apparent from the following detailed description.
For convenience, the following description will be directed to the metal plating of aluminum substrates, double zincating of aluminum substrates and electroless nickel phosphorous plating baths although it will be clear to those skilled in the art that other suitable metals and metal plating baths may be employed using the etchant and zincating compositions and methods of the invention to make metal plated aluminum substrate articles, including memory disks.